fix(internal/librarian): change log level to debug (#2798)

The directory removal is expected to fail if the directory is not empty.

Change the log level to debug so we know why the directory is not
removed in debug mode.

Fixes #2687

Author: JoeWang1127

internal/librarian/command.go

@@ -568,11 +568,10 @@ func clean(rootDir string, sourceRoots, removePatterns, preservePatterns []strin
 	var relPaths []string
 	for _, sourceRoot := range sourceRoots {
 		sourceRootPath := filepath.Join(rootDir, sourceRoot)
-		// If a source root does not exist, log a warning and searching the other source roots.
-		// TODO: Consider not calling clean if it's a first time generation
 		if _, err := os.Lstat(sourceRootPath); err != nil {
 			if os.IsNotExist(err) {
-				slog.Warn("unable to find source root. It may be an initial generation request", "source root", sourceRoot)
+				// If a source root does not exist, continue searching other source roots.
+				slog.Debug("unable to find source root. It may be an initial generation request", "source root", sourceRoot)
 				continue
 			}
 			// For any other error (permissions, I/O, etc.)
@@ -581,9 +580,8 @@ func clean(rootDir string, sourceRoots, removePatterns, preservePatterns []strin
 		}
 		sourceRootPaths, err := findSubDirRelPaths(rootDir, sourceRootPath)
 		if err != nil {
-			// Log a warning and continue processing other source roots. There may be other files
-			// that can be cleaned up.
-			slog.Warn("unable to search for files in a source root", "source root", sourceRoot, "error", err)
+			// Continue processing other source roots. There may be other files that can be cleaned up.
+			slog.Debug("unable to search for files in a source root", "source root", sourceRoot, "error", err)
 			continue
 		}
 		if len(sourceRootPaths) == 0 {
@@ -630,7 +628,7 @@ func clean(rootDir string, sourceRoots, removePatterns, preservePatterns []strin
 		slog.Debug("removing directory", "path", dir)
 		if err := os.Remove(dir); err != nil {
 			// It's possible the directory is not empty due to preserved files.
-			slog.Warn("failed to remove directory, it may not be empty", "dir", dir, "err", err)
+			slog.Debug("failed to remove directory, it may not be empty due to preserved files", "dir", dir, "err", err)
 		}
 	}